I also use CCI Magnums on the occasions I shoot percussion. I've never been able to tell any difference using various brands/types.
If you are speaking in regards to velocity differences you will not see any difference between brands or types as long as you are shooting black powder.
If you shoot one of the BP subs that are more difficult to ignite you will see differences. Between a regular percussion cap and the Remington Black Powder caps of 10 years ago you will see a spread of about 100 fps muzzle velocity in a .50 caliber patched ball rifle.
A "magnum cap" has about 20% more primer than a regular cap. The Remington Black Powder caps had twice as much primer as a standard cap. The difference was seen in velocities as the amount of primer went up so did the velocities.
An annoying part of that was the amount of primer residue left on the nipple and in the passage leading from the base of the nipple into the exit of the hole that carries the flame to the powder charge. More primer composition simply gives more residue after firing.
